Numerous glass enamel compositions that are lead-free and cadmium-free are known in the prior art.
In general, lead free glasses and lead free glass enamel compositions for glass coating systems reported in the prior art and prior literature can be categorized into two major, however rather broad, groups that are of interest to illustrate the importance and progression of the present invention. One group is often characterized as zinc-boro-silicate glasses. These types of glasses, even the ones claiming improved durability are often weaker in weathering the elements than their lead-containing counterparts. The other major group can be distinguished by its significant bismuth content. It is, therefore, very costly to produce this latter group, even though this group is often characterized as being better performing glass enamels. It is known to people familiar with these types of products that for decades the above-mentioned two major categories have been intended to displace the more forgiving, usually better performing, lead-containing materials which are now being phased out due to environmental and regulatory pressure.
An example of a prior art lead-free glass enamel may be found in Francel et al., U.S. Pat. No. 4,537,862, issued Aug. 27, 1985. Francel et al. discloses a glass enamel including a frit comprising by weight 25-32% SiO2, 25-32% B2O3, 10-18% ZrO2, 0-9% Na2O, 0-5% K2O, 0-9% Li2O, 0-2% SnO2, 0-3% CaO, 0-6% SrO, and 10-18% rare earth oxide, wherein the ratio of ZrO2 to rare earth oxide is about 1:1 to 1.4:1, the amount of ZrO2 plus rare earth oxide is about 20% to 33%, and the amount of Li2O, K2O and Na2O is about 1% to 10%. Another example of a prior art lead-free glass enamel may be found in Roberts, U.S. Pat. No. 4,859,637, issued Aug. 22, 1989. Roberts discloses a lead-free glass enamel including a frit comprising by weight 19-45% SiO2, 20-33% B2O3, 0-10% ZrO2, 8-15% alkali metal oxide, 0-35% ZnO, 0-6% Al2O3, 0-3% BaO, 0-8% P2O5, 0-6% La2O3, and 2-10% F2. Klimas et al., in U.S. Pat. No. 4,970,178, issued Nov. 13, 1990, teaches the inclusion of Bi2O3 in a lead-free glass enamel/glaze including a frit comprising by mole 45-60% SiO2, 6-13% B2O3, 0-6% ZrO2, 5-14% Na2O, 0-8% K2O, 0.0-5% Li2O, 8-25% ZnO, 0-8% CaO, 0-8% SrO, 0-10% Bi2O3, 0-4% Al2O3, 0-9% BaO, 0-7% TiO2, and 0-1% WO3.
Another example of a prior art lead-free glass enamel may be found in Emlemdi et al., U.S. Pat. No. 5,504,045, issued Apr. 2, 1996. Emlemdi et al. teaches that excellent scratch resistance and other desirable properties can be attained by a lead-free glass enamel including a frit comprising by mole 8.5-15.6% SiO2, 27.4-36.45% B2O3, 7.5-12.1% Na2O, 1.2-2.4% K2O, 0.0% Li2O, 1-2.6% MgO, 3.4-5.5% CaO, 0.5-1.4% SrO, 28.1-38.5% ZnO, 0.6-1.9% Al2O3, 0.9-2.2% P2O5, and 2.6-4.3% F2. A more recent example of a prior art lead-free glass enamel may be found in Kniajer et al., U.S. Pat. No. 6,346,493, issued Feb. 12, 2002. Kniajer et al. discloses a lead-free glass enamel including a frit comprising by weight 28-60% SiO2, 9-21% B2O3, 0-11% ZrO2, 0-15% Na2O, 0-13% K2O, 0-6% Li2O, 5-22% ZnO, 0-5% CaO, 0-2% MgO, 0-8% SrO, 0-9% Bi2O3, 0-6% Al2O3, 0-20% Nb2O5, 0-25% TiO2, 0-12% BaO, 0-6% Sb2O3, 0-5% LnOx, and 0-6% F2.